The present invention related to a method and device for improved position detection of the centroid of a beam of electromagnetic energy and, more particularly, to a device to detect the centroid of a focused beam referred to as an optical blur spot, obtained from a Bragg cell or acousto-optic (A/O) modulator or deflector. The device uses information from two or more photosensors to measure the centroid of the blur spot. This approach results in a simplified, highly-accurate, high speed measurement of a radio-frequency (RF) input signal to an A/O system.